1. Field of the Invention
This invention relates to wind turbine generator systems and multi-speed generation systems.
2. Description of the Prior Art
Large wind turbine generator (WTG) systems have, in the past, been typically confined to constant, single speed operation. Efficiency at constant speed, however, is limited since there is only a single wind speed at which maximum wind power can be extracted and useful energy, typically electrical, generated. Meteorological conditions typically dictate variable wind velocities daily and annually, and wind speeds above or below the single design wind speed value result in less efficient energy capture. The term energy capture refers to the amount of useful energy, for example electrical, generated as a percentage of the energy in the available prevailing wind.
More recently continuously variable or multi-speed wind turbine generators have been considered. For example, variable speed generation can be achieved through use of a variable frequency alternating current generator followed by a rectifier and an inverter. Variable speed transmissions have also been proposed. While such variable speed devices increase turbine efficiency, they do not necessarily increase overall system efficiency. Also proposed have been discrete speed conventional generators, such as two speed units, as well as hydraulic pump/motor combinations where a hydraulic motor drives a conventional generator.
While such systems offer increased turbine efficiency relative to single speed machines, they are not without deficiencies. For example, such systems are more complex and costly regarding design, manufacture, operation and maintenance. These complexities, including additional components and typically additional mass, are also carried through to support structures and other associated subsystems. System efficiencies can also be impaired due to additional losses such as in the rectifiers and inverters. Further, use of a conventional generator, or a conventional motor operating as a generator, involving multiple coils, one of which is typically not used at a given operational speed, makes inefficient use of fabrication materials.
It is therefore desirable to provide alternate wind turbine generator systems which achieve high energy capture and which further alleviate the complexities and inefficiencies associated with existing and proposed wind turbine generator systems.